Commercially available laser application devices (also called “laser markers” or “laser marking devices”) are developed based on a technology where characters, numbers, and symbols are thermally recorded on a medium such as a thermosensitive paper by the application of laser beams.
A laser beam emitted from a laser light source of the laser application device is applied to media such as plastic or thermosensitive paper, such that characters, symbols, etc., are recorded on such media. Examples of the laser light source include a gas laser, a solid-state laser, a liquid laser, and a semiconductor laser (i.e., a laser diode, LD).
With metallic or plastic media, heat generated upon the application of a laser beam engraves or singes surfaces of the media, so that characters and symbols are recorded on the metallic or plastic media. On the other hand, the thermosensitive paper has a characteristic of changing its colors with heat, and the heat generated upon the application of a laser beam causes recording layers of the thermosensitive paper to generate colors. Accordingly, characters and symbols are recorded on the thermosensitive paper.
Compared to the metallic or plastic media, the thermosensitive paper is relatively easy to handle and hence is widely used as labeling media. For example, distribution destinations of articles or article names are recorded on the thermosensitive paper used as labeling media in the physical distribution field.
Recently, rewritable thermosensitive paper (hereinafter called “thermal rewritable media” or “thermoreversible recording media”) have been made available, on which information can be repeatedly recorded or erased.
At present, with the thermoreversible recording media, images are recorded on or erased from the reversible recording medium by direct contact with a heat source (i.e., a contact recording/erasing process). In this case, a thermal head is generally used as the heat source in image recording, and a thermal roller, ceramic heater, and the like may be used as the heat source in image erasing.
Such a contact recording/erasing process has the following advantages. That is, when the thermoreversible recording medium is a flexible medium such as a film or paper, the flexible recording medium may be uniformly pressed against the heat source with a platen to uniformly record images on or erase the images from the flexible recording medium. In addition, since components of a conventional printer specifically used for thermosensitive paper may be diverted to become components of a new image recording apparatus or those of a new image erasing apparatus, manufacturing costs of the new image recording or image erasing apparatus may be reduced.
FIG. 1 is a diagram illustrating a coloring/decoloring principle in a thermal rewritable medium.
The thermal rewritable medium includes a recording layer that reversibly changes its color tone into a transparent status or a colored status with heat. The recording layer includes leuco dyes of organic low-molecular substances and reversible developers (hereinafter simply called “developers”).
As illustrated in FIG. 1, when the recording layer in a decolored state A is heated to a melting temperature T2, the leuco dyes and the developers in the recording layer are melted and mixed so that the recording layer is colored in a melted colored state B. When the recording layer in the melted colored state B is rapidly cooled, a temperature of the recording layer is decreased to room temperature while the recording layer maintains its colored state, thereby stabilizing the colored state of the recording layer. Accordingly, the recording layer is in a solid colored state C. Whether the recording layer is capable of obtaining such a solid colored state C depends on a speed of heating or cooling the recording layer in the melted colored state B. If the recording layer is slowly cooled, the recording layer is decolored to be in the initial decolored state A. If, on the other hand, the recording layer is rapidly cooled, the recording layer acquires a relatively dense color compared to that in the solid colored state A.
Meanwhile, when the recording layer in the solid colored state C is heated again, the recording layer is decolored at a temperature T1 lower than a coloring temperature (from D to E), and if the recording layer in the solid colored state C is then cooled, the recording layer returns to the initial decolored state A.
In the recording layer in the solid colored state C changed from the melted state by rapid cooling, colored leuco dye molecules and developer molecules are mixed while they remain in contact reactive states, and the molecules in the contact reactive states often form solids. In this state, the melted mixture (colored mixture) of the leuco dyes and developers is crystallized while retaining its colored state, and hence, the color of the mixture may be stabilized due to this crystallized structure. In the decolored state, phases of the leuco dyes and the developers are separated from one another. In the phase-separation state, the molecules of one of the leuco dye compound and the developer compound are cohered or crystallized. Accordingly, the leuco dyes and the developers are separately stabilized due to their cohesion or crystallization. In many cases, more complete decoloration (color erasure) of the recording layer may be obtained due to the phase separation of the leuco dyes and the developers, and crystallization of the developer.
Note that erasing failure, where the recording layer repeatedly heated at an erasing temperature is unable to decolor, may occur if the recording layer is repeatedly heated to a temperature T3 that is equal to or higher than the melting temperature T2. The erasing failure may result from thermal decomposition of the developer, because the thermally decomposed developer is resistant to cohesion or crystallization and thus the thermally decomposed developer may not be easily separated from the leuco dye. Deterioration of the thermoreversible recording medium due to repeated heating and cooling may be controlled by decreasing the difference between the melting temperature T2 and the temperature T3 while heating the thermoreversible recording medium.
Such thermoreversible recording media are widely used in the physical distribution field, and various improvements have been made to a recording (marking) method in the recording of the thermoreversible recording media.
For example, when adjacent first and second lines are marked, the residual heat of the first line initially marked may interfere with the heating of the second line while the second line is being marked. This interference may result in decoloration of the recording in the thermoreversible recording media. Japanese Patent Application Publication No. 2008-62506 (hereinafter referred to as “Patent Document 1”), for example, discloses a technology in which such decoloration is controlled by adjusting a time or an overlapping width between a marking start point of the first line and a marking end point of the second line.
However, if the thermoreversible recording medium contains an RF-ID tag, the thermoreversible recording medium has an increased thickness and thus is less flexible. Accordingly, higher pressure may be required when the heat source is uniformly pressed against the thermoreversible recording medium (see Japanese Patent Application Publication No. 2004-265247 (hereinafter referred to as “Patent Document 2”) and Japanese Patent No. 3998193 (hereinafter referred to as “Patent Document 3”)).
Further, if printing and erasure are repeated in such a contact recording/erasing process, the recording medium obtains an uneven surface due to ablation. Accordingly, erasing failure or density decrease may occur due to non-uniform application of heat resulting from portions of the recording medium not contacting the heat source such as a thermal head or hot stamp (see Japanese Patent No. 3161199 (hereinafter referred to as “Patent Document 4”) and Japanese Patent Application Publication No. 9-30118 (hereinafter referred to as “Patent Document 5”)).
Japanese Patent Application Publication No. 2000-136022 (hereinafter referred to as “Patent Document 6”) discloses a technology in which an image is uniformly recorded on and erased from an uneven surface of the thermoreversible recording medium using a laser, or an image is uniformly recorded on the thermoreversible recording medium from a distance using a laser. This technology is used for transportation containers. With this technology, contactless recording is performed on thermoreversible recording media that are attached to the transportation containers, where recording is carried out by laser beams but erasure is carried out by hot air, hot water, or an infrared heater.
Inspired by contactless reading or contactless rewriting of recording information performed on RF-ID tags from a distance, there is a desire that images also be rewritten on the thermoreversible recording media from a distance.
In such a laser recording technology, a laser recording device (generally called a “laser marker”) is used. The laser marker is configured to control a laser beam such that the laser beam is applied to an appropriate position of the thermoreversible recording medium when the laser marker applies a high-power laser beam to the thermoreversible recording medium. In this laser marker, the thermoreversible recording medium absorbs a laser beam and converts the absorbed laser beam into heat, so that information is recorded or erased by the converted heat. Japanese Patent Application Publication No. 11-151856 (hereinafter referred to as “Patent Document 7”) discloses a laser recording-erasing technology in which images are recorded on or erased from the thermoreversible recording media, and formed based on a combination of a leuco dye, a reversible developer, and various photothermal conversion materials by the application of infrared laser beams.
Further, Japanese Patent Application Publication No. 2008-213439 (hereinafter referred to as “Patent Document 8”) discloses an image processing method (image marking control method) in which when laser beams aligned at predetermined intervals are applied in parallel to scan in the same directions, discontinuous application of the laser beams may be partially included. For example, when the laser beam scans from a first starting point to a first ending point, the laser beam is caused to scan a second starting point by jumping from the first ending point to the second starting point where the first ending point and the second starting point are separated by a predetermined interval (gap).
With above related art technologies, the thermoreversible medium may be uniformly heated so that image quality and repeated durability of image formation on the medium are improved; however, image recording or erasing time may be increased due to time required for jumping across drawing line intervals and waiting time during jumping.
Moreover, Japanese Patent No. 3557512 (hereinafter referred to as “Patent Document 9”) discloses a technology in which a laser beam scans in a looped or a convoluted fashion so as to apply the laser beam to an entire cell region. In this case; however, excessive heat is applied to curved portions of the loop or convolution so that the repeated durability of image formation in the thermoreversible recording medium is lowered.
Thus, in the related art technologies, there may be few image recording technologies capable of printing with high printing quality and high repeated durability of image formation, and recording an image on a medium in a short time; or there may be few image erasing technologies capable of uniformly applying heat to the recording medium, acquiring a wide erasing width of the medium, and erasing the recorded image in a short time.
The above related art technologies may include the following drawbacks.
The laser marker generally scans plural lines in parallel to fill an area with a solid color. However, with a thermal rewritable medium, simply scanning plural lines in parallel may not achieve the solidly filled color.
As illustrated in FIG. 1, the thermal rewritable medium has the decoloring temperature between the room temperature and the coloring temperature. Thus, when the thermal rewritable medium is heated with a laser beam, peripheries of the marked lines become decoloring temperature regions of the rewritable medium due to output intensity distribution of the laser spot or thermal diffusion on the rewritable medium. Further, if a coloring property of the rewritable medium is broad with a temperature, the densities of the scanned lines may not be uniform in width directions of the scanned lines. In order to fill an area with the uniform density, a line is marked by slightly overlapping a previously marked line such that a residual heat region in the periphery of the previously marked line is cancelled (see Patent Document 1).
Note that since the residual heat of the scanned line decreases with time, it is important to control time intervals in marking lines using the residual heat. In view of discoloring due to accumulated heat, if a marking speed is high or a marked line is short, it is preferable that a time interval in marking lines be long, whereas if the marking speed is low or the marked line is long, it is preferable that a time interval in marking lines be short.
FIGS. 2A, 2B, and 2C are diagrams illustrating a marking method in which a subsequent line is marked (scanned) by partially overlapping a previous line. FIGS. 3A and 3B are diagrams illustrating a marking method in which lines are marked by reciprocating scanning.
In FIG. 2A, flat ovals indicate a profile of coloring line, solid line arrows indicate operations of marking (marking operations), and broken arrows indicate jumping operations (non-emitting operations) between marking points, so that an area is filled with a solid color by repeating the following steps 1 to 3.
Step 1. A laser marker is illuminated to scan a line from a first starting point in a plus direction of an X-axis with a predetermined laser power at a predetermined speed.
Step 2. The laser marker is turned off and moved to a second starting point (in a minus direction of a Y-axis direction).
Step 3. The laser marker waits for a predetermined waiting time.
Accordingly, the area is filled in solid as illustrated in FIG. 2B. In the marking method illustrated in FIG. 2A, since the laser marker carries out jumping operations (non-emitting operation) and waiting while repeating to mark marking lines in the plus directions of X-axis as illustrated in FIG. 2C, solidly filling an area with the marking lines may require a long time. However, if thick lines such as a barcode are marked, the above method may be necessary for solidly filling the area with the marking lines. Thus, a higher marking speed may be required for shortening the marking time.
In the example of solidly filling the area illustrated in FIGS. 2A, 2B, and 2C, the lines are repeatedly marked in the same directions to solidly fill the area, so that the laser marker (or beam) needs to jump (carry out a non-emitting operation) a distance corresponding to a length of the marked line or longer. Thus, this method is not suitable for high speed marking.
However, in ink-jet printers, it is commonly known in the art that lines are marked in two opposite directions while reciprocating a printer head in the two directions. Accordingly, the lines may be marked by a laser marker at higher scanning speeds with a reciprocating scanning method illustrated in FIG. 3A.
However, when the marking direction is reversed (i.e., opposite direction), marking is started with a side where marking of the previous line is finished. Thus, the laser marker needs to have a longer waiting time for allowing the residual heat to dissipate, which may not contribute to a reduction in an overall marking time as illustrated in Patent Document 1.
Further, an effect of the residual heat of the previously marked line is greater in the next starting point of a subsequent marking line and smaller in the next ending point of the subsequently marked line. Accordingly, color variability may be obtained when the lines are marked with a weak (low) laser power as illustrated in FIG. 3B. On the other hand, if the lines are marked with a high (strong) laser power, the area may be uniformed filled with solid lines without color variability. However, if marking and erasing are repeated, the starting point portions with high densities illustrated in FIG. 3B may be some quickly deteriorated and thus these portions of lines may no longer be erasable.
The inventors of the present invention have found the following based on various examinations.
A related art method illustrated in FIGS. 4A, 4B, and 4C is used as a laser beam scanning method for solidly filled image printing and solidly filled image erasure using a round laser beam. In FIGS. 4A to 4C, a laser marker scans thick solid lines with laser beams at uniform speeds, whereas the laser marker carries out non-emitting operations indicated by broken lines without emitting a laser beam.
A laser scanning method illustrated in FIG. 4A is capable of scanning a laser beam in a short time so that a solidly filled image printing or solidly filled image erasure may be carried out in a short time. However, excessive energy may be applied to the medium due to effects of a (low) scanning speed of a laser beam in turning portions of the lines and heat accumulated in the turning portions of the lines. These effects are obtained by marking a starting point of a second laser beam marking line 412 immediately after marking an ending point of a first laser beam marking line 411. Accordingly, density (color) variability (density lowered in the image portion and color appearing in the image erased portion) may be obtained in the solidly filled image portion or in the image erased portion on the medium.
A laser scanning method illustrated in FIG. 4B is capable of scanning a solidly filled image printing in a short time with little effect of lowering a scanning speed at turning portions of the lines. However, there is still the effect of heat accumulated in the turning portions of the lines obtained by marking the starting point of the second laser beam marking line 421 immediately after marking the ending point of the first laser beam marking line 422, such that excessive energy may be applied to the medium. Accordingly, density (color) variability (density lowered in the image portion and color appearing in the image erased portion) may be obtained in the solidly filled image portion or in the image erased portion of the medium. Further, the repeated durability of the solidly filled image may be lowered.
Moreover, laser scanning method illustrated in FIG. 4C eliminates adverse effects of the scanning speed and the heat accumulation in the turning portions of the lines, such that excessive energy may not be applied to the medium. Accordingly, the density (color) variability may not be obtained in the solidly filled image portion or in the image erased portion of the medium. Further, the repeated durability of the solidly filled image may be improved. However, with this method, time for non-emitting portions (no laser application) may be longer, thereby increasing the image printing time and image erasure time. In addition, with this method, since the effect of heat accumulation in the turning portions of the lines is lowered, the second laser beam marking line 432 is marked or erased in a cooled state after the scanning of the first laser beam marking line 431. Accordingly, the accumulated heat is not utilizable for marking or erasing of the second laser beam marking line 432. As a result, high energy is required for the marking or erasing of the subsequent laser beam marking lines. Thus, the solidly filled image printing time or image erasure time may not be reduced due to inability to increase the scanning speed.
However, as illustrated in FIG. 5B, if the laser beam marking line 451 is marked and scanned from the first starting point to the first ending point, and the second laser beam marking line 452 is subsequently marked adjacent to the first laser beam marking line 451 from the second starting point to the second ending point such that the second ending point of the second laser beam marking line 452 is located in a line slanted toward the first starting point of the first laser beam marking line 451 based on a line in parallel with the first laser beam marking line 451, density variability in the solidly filled image area (portion) or the image erased area (portion) may be suppressed. Accordingly, the repeated durability of the solidly filled image may be improved and the solidly filled image printing time or image erasure time may be lowered.